The level and strength of cell service in an area determines what functions you have access to on your Cell Phone.
Full service is found in cities, as well as most towns and major roadways connecting them. Full service supports all functions of the smartphone.
Smaller and more rural towns don't always have the most recent technology. In these towns you'll have limited service, which supports only basic functions like calling and texting.
In remote areas between cities, your phone service may have a weak signal. Weak service supports all smartphone functions, but your battery discharges at twice the usual rate as your device must use more energy to communicate with the tower.
With service that is both limited and weak, only basic calling and texting functions are supported and battery usage is doubled. Try to minimize phone usage in these areas to conserve your battery in case you need to call for help.
In the far reaches of the wilderness, or when you're underground, you'll have no signal at all. You can't use any of your phone's functions that require service, but you can still use localized functions like the flashlight and clock.